This invention relates to electric lamps and, in particular, to an improved composite lead wire for use in electric lamps.
In the prior art, copper and various copper alloys have been used as lead wire material in electric lamps. A recurring problem has been the weakening or annealing of the inner portion of the lead during glass forming operations.
In an effort to forego the use of tie wires and the related manufacturing necessities, dispersion strengthened copper lead wires (DSC) have been used. U.S. Pat. No. 4,138,623 teaches the use of DSC wire, normally a "Glidcop" AL-20 or its equivalent containing 0.20% aluminum oxide calculated as the metal equivalent, with a thin copper plating or sheath generally measuring a fraction of a millimeter in thickness, surrounding an inner core of internally oxidized dispersion strengthened copper. The DSC wire may further be nickel plated to reduce the release of contaminants from the underlying copper. This technology afforded manufacturers an opportunity to eliminate the use of the tie wires to support copper or copper alloy lead wires.
In U.S. Pat. No. 4,208,603, it is taught that if the copper plating or sheath is removed from DSC wire, and the nickel is plated directly onto the dispersion strengthened copper, the bonding of the nickel onto the DSC is enhanced, thus reducing problems resulting from nickel migration during lamp operation which results in filament brittleness.
In U.S. Pat. No. 4,415,830 it is suggested that iron alloys or steel are also suitable lead wire materials when containing a high silicon content, between 2 wt. % and 4.5 wt. %, well in excess of normal trace amounts of silicon in iron alloys, and having a carbon content of between 0.01-0.02 %. This material is taught to avoid allotropic transformation of the alpha ferrite, body-centered cubic, crystalline phase at lamp operating temperatures. During this phase transformation, the microstructure of the iron alloy changes in response to temperature increases during lamp use. The wire returns to its original phase during non-use. The constant phase change in the lead wire eventually causes the filament clamp to loosen, causing lamp failure. The '830 patent avoids this problem by using high silicon content iron alloy or steel wire to control allotropic transformation. It is necessary to maintain the carbon content of the wire at a low level, about 0.01-0.02 wt. %, because at increased levels of carbon the amount of silicon necessary to counteract the phase transformation becomes unworkable. Copper plating is used in this reference to prevent iron contamination.
Another drawback of steel lead wires is their low electric and thermal conductivity. Because of this, the steel lead wires have larger diameters, on the order of 20 mils. Thick lead wires, however, can cause problems in manufacturing and sealing the lamp, as well as cause an increase in the expense of producing the lamp.
The foregoing technology, while presenting viable options, does not completely solve lead wire problems relating to high temperature processing integrity of the lead wires, or reduced manufacturing expense without a corresponding reduction in lamp performance. It has remained for the subject invention to disclose the use of steel wire having a thick copper cladding, as opposed to a sheath or plating, which generally refer to a thin coating, for use as lead wire in lamp applications.